1. Field of the Invention
This invention relates to electrical connectors for infrared detectors and, more particularly, to arrangements for improving the reliability of connections to a plurality of sensors in a detector array assembly which is subject to thermal fatigue from temperature cycling.
2. Description of the Related Art
In the present fabrication of focal plane arrays for infrared sensing systems, the hybrid detector array assembly comprises a pair of microchips, one bearing the array of sensors and the other bearing a corresponding array of cells or diodes with associated contact pads to provide the readout of individual sensor signals. The contact pairs of the two microchips are joined together in a process called hybridization. In this process, a plurality of indium bumps on the detector chip and a corresponding plurality of indium bumps on the readout chip are cold welded together by pressure. Once joined, they are no longer separable and the breaking of any weld constitutes a failure of that readout cell.
Over time an infrared detector array is repeatedly cycled between room temperature and its normal operating temperature of 77 degrees K. This repeated temperature cycling is responsible for problems relating to thermal fatigue which results from the different coefficients of thermal expansion in the different materials present in the hybrid detector assembly.
In the present (prior art) fabrication process, the indium bumps are made by vapor deposition through a photo-reduced mask pattern and have a typical height of 6-9 microns. It is not possible to deposit the indium bumps more than 10 microns high with acceptable quality and density. Over the temperature cycling range between room temperature and the 77 degree K. operating temperature, the various materials present in the array account for the thermal fatigue problems. For example, the readout chip is a silicon substrate with contact pads approximately 0.001 inch square on 0.002 inch spacing. A typical array may have 128.times.128 cells. The sensors are arranged in a similar array on a cadmium telluride substrate. Because of the differences in thermal expansion and contraction between the detector chip and readout chip, repeated temperature cycling results in various failure modes: contact pads are pulled away from the substrate, pieces of contacts break off, the cold-welded junctions of the indium bumps fracture and separate, the stresses induced by the differential thermal expansion or contraction of the substrates may cause warpage of the array chips, and the like. It is known that these problems could be minimized somewhat if the indium bumps, prior to being welded together, were made taller or longer so that they could still establish the desired circuit connections while increasing the spacing between the detector chip and the readout chip. The resulting more compliant arrangement is expected to exhibit more tolerance for the effects of thermal expansion and contraction during repeated temperature excursions between room temperature and operating temperature. However, no simple way of building up the indium bumps to increase their length has been proposed heretofore, to our knowledge.